Slide-bead coating is a process well known in the art. It entails flowing a liquid layer or layers down an inclined slide surface to an efflux end, or lip, positioned a short distance from a moving substrate. The liquid forms a bridge, or bead, in the gap between the lip and the moving substrate. The moving substrate carries away liquid from the liquid inventory in the bead in the same layered structure established on the slide. See, for example, Russell, et al., U.S. Pat. Nos. 2,761,791 and 2,761,419.
For a given coater arrangement, coating liquid, and flow conditions, the range of applied differential pressures giving a satisfactory coating is a function of substrate velocity and is limited by the onset of bead instabilities and/or other practical considerations. As described by Saito, et al, in "Instability of the Slide Coating Flow" 1982 Winter National AIChE Meeting, Orlando, Fla., the coating bead becomes unstable giving rise to evenly-spaced disturbances in the subsequent coating when the substrate surface velocity and/or differential pressure are too high. On the other hand, if the differential pressure is too low, the width of the bead decreases undesirably and/or the bead becomes unstable thereby creating a coating which is too narrow and/or is disturbed. The unsatisfactory results from a differential pressure that is too high or too low define an operating window which is herein called a useful differential pressure range for producing coatings of satisfactory quality and width.
Unfortunately, operation within the useful differential pressure range alone does not guarantee a satisfactory coating. Surface and fluid dynamic forces, particularly at the lip surface, also affect the coating quality. In the lip region of a conventional coater, as shown in FIG. 2, the lip surface, 17, is typically longer than 0.5 mm. Immediately after coating is started, a static contact line, 18, of the bead forms at some location along this lip surface, 17. The location of the static contact line, 18, is typically from 0.05 to 0.50 mm below the lip tip, 16, and is determined for a given lip geometry by a balance of forces that can be resolved into fluid dynamics, applied differential pressure, and local surface forces acting along the lip surface, 17.
The effect of applied differential pressure on the static contact line position, 18, and the dynamic contact line position, 19, can be observed, correlated and predicted. Increasing the differential pressure will result in static contact line movement to say position 18'. Decreasing the applied differential pressure will result in static contact line movement to say position 18".
The effect of the surface and fluid dynamic forces on static contact line position 18 is more complex and difficult to predict. In the vicinity of the static contact line, these forces tend to be weak and therefor do not dictate a strongly preferred location for the static contact line under a given set of operating conditions. Consequently, when establishing a new static contact line, any nonuniformity in either the surface or in the transient flow can result in an irregularity in the contact line straightness across the transverse extent of lip 17. Such static contact line irregularity interferes with the uniformity of the bead and can generate an undesirable variation in the thickness of the coating across the substrate. These thickness defects, often called streaks in the coating art, may render the resulting material unusable for the intended application. Surface nonuniformities leading to static contact line irregularities include local deposits from the coating solution, substrate-, liquid- or gas-borne foreign matter, lip surface contaminations and physical damage.
Various lip surface modifications have been proposed to avoid the occurrence of the streak defects. In Kitaka and Takemasa, U.S. Pat. No. 4,440,811, the coater lip region is modified to include a notch whereby the bead contact line is preferentially located along the notch tip. However, the proposed configuration is expensive to fabricate to the precision required and, in practice, the notch is difficult to clean and promotes deposits and settling from the flowing material. In addition, most configurations incorporating the notch produce lip tip-to-substrate gaps that are larger than the narrowest mechanical gap by a length equal to the extent of the notch. This arrangement undesirably results in a reduced maximum useful differential pressure. The decrease in operational latitude translates into a decrease in the absolute range of differential pressure within which bead uniformity is maintained and may also reduce the achievable coating speed which decreases overall productivity of the coating apparatus.
Japanese Patent Publication No. 48-4371 discloses use of a land inclined with respect to the substrate tangent so as to locate the wetting line at the sharp coating lip. The sharp lip region is excessively vulnerable to mechanical damage such as a crack or scratch that would cause streaks in the coating. To avoid this problem, U.S. Pat. No. 3,928,678 discloses rounding or bevelling the lip tip to increase mechanical strength of the lip tip and move the bead static contact line away from the lip tip. But, no dimensions or orientations are disclosed for maintaining the bead static contact line at a preferred or advantageous position. As stated by Hitaka et.al., in U.S. Pat. No. 4,440,811 in reference to using such a bevel: " . . . it was difficult to hold the end of the beads at a fixed place or to restore the said end to the original state." Furthermore, the bevel depicted in U.S. Pat. No. 3,928,678 results in a larger lip tip-to-substrate gap than the narrowest mechanical gap with the attendant loss in maximum useful differential pressure.